Heater elements applied to glass or other nonconducting substrates can be used as radiative heater units if the installed heating power is adequate for this purpose. These heater elements can be installed on or in building walls or be integrated into these buildings as a replacement for the usual (central) heating units. For this purpose, they do not need to be implemented in the form of windows but can equally well be in the form of mirrors, decorative surfaces, etc. Alternatively, it is also possible for such panel elements to be used as a general means to produce heating from the surfaces of technical appliances, for example domestic appliances, where their limited installation height and smooth, easily-cleaned surfaces offer big advantages.
The power requirements of large-area heating layers demand the use of relatively high voltages. Safe and reliable electrical isolation must therefore be provided, particularly along the edges of the panel concerned, which may be coated over its whole surface.
DE-A1 198 60 870 describes a panel heater element of this type, with a glass substrate and its whole surface coated. In order to ensure safe and reliable isolation from the outside of the electrically powered coating, a region forming a frame around the periphery of the coating is isolated by a separation line and is thus electrically neutralized. Such an arrangement also protects the coating from corrosion that penetrates in from the outer edges, but that can only penetrate as far as the separation line.
The electrical connections to the heating layer are connected to the coating situated on the inside of the surface surrounded by this frame, other separation lines defining a heating current path over the whole surface of the coating. The same document also discloses the option that consists in coating with an electrical conductor two or more of the bonded panes of a laminated or safety glazing panel. The details of the practical implementation of such a laminated pane will not however be entered into here, either as regards the electrical connections or the electrical control of such a double heating layer.
In another known panel element (document DE-B-2 113 876), the electrically conductive heating coating does not extend right up to the edge of the panel, such that a spacer frame for an insulating glazing panel can be bonded, without any special arrangements, directly onto the border (where there is no layer) of the glazing panel. The electrode power supply conductors pass through sealed feed holes formed in the spacer frame. The second pane of the insulating glazing panel has a non-heating solar protection layer.
The prior patent application 102 41 728.8 of Applicant describes a connecting device for a laminated panel element that comprises a first rigid pane having a heating layer together with a second rigid pane joined over its whole surface with the first by adhesion. The connection device is inserted into a feed hole formed in one of the rigid panes. It comprises contacts that allow a direct contact with the heating layer to be established. For this purpose, the latter has at least two electrodes that are disposed in the region of said cut-out. A plurality of current paths, electrically connected in parallel and formed within the coating, can run between these electrodes.
The length and width of the current path or paths together with the surface conductivity (expressed in ohms per square) of the layer configuration used will determine the consumption of electrical energy and the heating power of the panel element. Depending on the particular power supply voltage available or predetermined, various heating powers can be established over wide ranges by the current path pattern, the maximum admissible temperature also depending on the type of use of the finished panel element. If, for example, it is not possible for the user to come into direct contact with it, or this need not be assumed, the temperatures can be well above 50° C. However, care must of course be taken to ensure that coatings adhering to the coated pane, for example adhesive layers of a laminated glazing panel, are not degraded at the temperatures that may be reached during normal operation.
In the literature, various materials are mentioned that are suitable for such heating layers. Solely as examples, indium tin oxide (ITO) and metals that are good conductors such as gold, silver, copper or aluminum are mentioned here. Layer configurations having dielectric antireflection layers and at least one metallic layer situated between them allow very good transmission of visible light with satisfactory electrical conductivity, but can also be used, at the same time, as infrared reflectors. Such typical layer configurations exhibit sheet resistances in the range of 1 to 4Ω per square.
When these heater elements operate at high heating powers, the adhesive layer, generally a thermoplastic (preferably, a PVB, PMMA or EVA sheet), can reach its thermal limits. The adhesion to the (coated) glass surfaces may fail when the full heating power is applied for a sufficiently long period. In some cases, and in particular at high current density locations, this can lead to local delaminations of the coating. Since, for reasons of production method and cost, moving away from the adhesives tried and tested over several years in laminated glass manufacturing is undesirable, other means of avoiding these thermal problems must be found.